Radiation machines such as medical linear accelerators are useful in producing high energy radiation to treat patients with cancer. Depending on the type of cancer, position, size of the tumor and its surrounding critical organs, and the patient size, medical linear accelerators may operate at high energies ranging from about 4 MV to about 20 MV for radiation therapy procedures. To ensure safety, protective measures such as various radiation shielding must be taken to limit unwanted radiation to patients outside the planned treatment field and to radiotherapists and the general public to an acceptable level.
FIGS. 1A and 1B schematically show a conventional beehive or shielding structure 10 for a linear accelerator. The beehive 10 includes plural rows e.g. six rows as shown of lead stacked vertically to surround an accelerator guide. Each of the stacks is further cut into multiple pieces e.g. two to four pieces as shown to keep the weight manageable for someone to install or remove the accelerator guide. In the conventional beehive 10 shown in FIGS. 1A and 1B, each of the gaps and bolt clearance holes represents a potential radiation leakage path. Further, removing or installing an accelerator guide requires removal of all the heavy pieces of lead, which is labor intensive and can be dangerous as the guide must be removed while oriented in the vertical position high above the floor (about 8 to 9 feet high).
In conventional production implementation shown in FIG. 1C, the magnetic shield 12, a part serving to shield an accelerator guide 14 from variations in earth's magnetic field during rotation, is a separate piece part installed around the guide 14 prior to adding the lead shielding pieces described above. Mechanical alignment of the guide 14 with respect to the isocenter is done using a tripod feature 16. The tripod feature 16 extends radially outside the base 11 of the beehive 10 so that access to the jacking screws 18 and retention bolts 20 is possible. When alignment of the guide 14 is performed, the guide 14 is tilted and shifted laterally by adjusting the screws 18 and bolts 20 on the tripod feature 16 and lateral adjustment bolts 22 on the head frame 24. During the alignment, the lead beehive (10 in FIG. 1A) remains stationary while the guide 14 and tripod 16 translates or tilts. One limitation of the conventional design and implementation is that it requires mechanical clearance to avoid collisions of the guide 14 with the beehive 10 and magnetic shield 12 etc. The added clearance results in larger clearance holes for the tripod feature 16 which must be blocked with additional shielding extensions to the beehive 10. The increased clearance also causes increased shielding diameter to provide the required attenuation, which increases cost, weight, and volume taken up in the gantry.